The present invention is directed to communication systems for digital information and particularly to an electronic digital switching system for data communication which is suitable for use with different types of remote digital terminals.
Particularly, the invention is directed to a conversion circuit arrangement for switching digital encoded signals between different types of data terminals operating with distinctive interfaces with respect to data transmission procedures and to a method for switching digital encoded signals between such data terminals of different types. The term "data transmission procedure" is hereinafter referred to including the signalling information and procedure, the control of data transmission and also to the data format and the data rate during transmission.
Data communication networks, such as the telex network, have become increasingly important during the recent years. Data processing techniques have penetrated all branches of industry, commerce and public administration and, as a result, different types of data terminals using different transmission procedures have been developed. Office teletypewriter machines are an example of a new kind of remote data terminal which will come more and more into use. This type of a data terminal combines facilities for typewriting, text processing and telecommunication and is, in general, composed of a keyboard, a display, a printing unit, storage units, and a transmission unit. The main concept is to have reasonably priced office equipment combining all of these different facilities and to have this equipment connectable to public communication networks so that the equipment operator can communicate with operators of similar equipment. Under this concept, even telex stations are connectable to such office teletypewriter stations, also often called teletex machines. The usage of terms with respect to this new type of data terminal is not yet consistent. For distinguishing the conventional teleprinter from this office equipment having the described characteristics hereinafter the term "teletex stations" is used. Accordingly, facilities for two different data transmission procedures (i.e. standards) have to be provided. A telex station operates in a start-stop mode and with a data rate of 50 bits/sec., whereas a teletex station connected to a telephone system has synchronous data transmission at a rate of 2,400 bits/sec.
Recently developed switching systems for digital telecommunications are suitable for use with different communication networks to handle telex traffic and other data traffic. A digital switching system of this type is disclosed in the technical manual Siemens Systems EDS, System Description, which is published and distributed by Siemens Corp., Iselin, N.J., and which is incorporated herein by reference. This known electronic digital switching system is capable of handling the above-mentioned different types of data traffic with one exchange, that is the hardware and software is designed to respond to different data transmission procedures for telex traffic and other data traffic simultaneously. However, the constraint of having a complex structure for different signalling, data format and data rate restricts the use of one exchange to switching operations within a respective one of the groups of data terminals. For this reason, basically the data switching system has to have data terminal interfaces so-called line adapters which are grouped in sets each serving a different type of data terminals. Two line adapter sets, each assigned to a different group of data terminals operating with different data transmission procedures, cannot be linked together to set up a physical communication link across the exchange. Nevertheless, an electronic digital switching system serving different types of data traffic is a sophisticated special purpose data processing system; with extensive software and even built-in hard wired logic required for real time conditions and system reliability.
Trends of current data traffic and design features meeting requirements of increasing communication needs and offering possibilities for integrating the up to now independent data networks have been discussed in an article in Nachrichtentechnische Zeitschrift, Vol. 22, 1969, pages 444 to 463 with the title "EDS-A New Electronic Data Switching System For Data Communication" which article is also incorporated herein by reference. The contents of this article point out very clearly the complexity of design of such an electronic digital switching system which is necessary to meet recent requirements. It is understood by those skilled in the art that such a special purpose data processing system for handling data transmission procedures reasonably cannot be designed to be as flexible as to offer all possibilities of direct communication between different data terminals. It is nearly impossible to be prepared for new data terminals which may become important at some future time after the exchange is already installed.
Coming back to the aforementioned example of direct data communication between a teleprinter and a teletex station, the present restriction in data exchange of switching data just from one type of data terminal to the same type of data terminal requires that teletex stations be designed such that they can be switched between two modes of communication and operation. In particular, a teletex station must be designed to operate at a data rate of 50 bits/sec. using a start-stop method or at a rate of 2,400 bits/sec. for example, in a synchronous mode of operation. In the individual teletex station, however, this dual transmission capability means the provision of additional circuitry with its additional cost.
It is therefore, one object of the present invention to overcome restrictions of data communications in known data communication systems.
Another object of the present invention is to provide apparatus, adapted for use with an electronic data switching system, which avoids the need for expensive switching equipment in remote data terminals while permitting such data terminals to communicate with data terminals operating with different data transmission procedures.
Still another object of the invention is to improve known data switching systems to achieve more flexibility in data communications between data terminals of different types without the cost of special circuitry.